Light-emitting diode (LED) light sources are becoming more common in the marketplace. Originally used to replace conventional incandescent, fluorescent, or halogen lamps, and the like in homes, LED light sources are now becoming more commonly used in products such as automobiles. Their increased use is not surprising as LEDs are typically more efficient than conventional incandescent light bulbs and the like, and have longer operational lives.
In some implementations, LED light sources include a plurality of light-emitting diodes provided in a suitable housing. LEDs are current-dependent components, and thus in order to illuminate LEDs properly an LED driver control device (an LED driver) is typically coupled between an alternating-current (AC) source and the LED light source to regulate the power supplied to the LED light source. The LED driver may regulate the current supplied to the LED light source to a specific peak current value, or regulate the voltage provided to the LED light source to a particular value, or may regulate both the current and the voltage.
Many different LED driver configurations are known, and some provide power by using pulse-width modulation (PWM). Some driver circuit designs switch an LED power supply unit on and off using a pulse duration modulator to control the mean light output of the LEDs. Thus, PWM signals may be used to alter the brightness and color of LEDs.
The light output of an LED is proportional to the current flowing through it, and thus methods have been developed to control the current delivered to an LED light source. For GaInN type LEDs, a typical load current is about 350 milliamps (mA) at a forward operating voltage of between three and four volts (3V-4V), which corresponds to about a one watt (1 W) power rating. At this power rating, this type of LED provides about 100 lumens per watt which is significantly more efficient than conventional light sources. For example, incandescent lamps typically provide 10 to 20 lumens per watt and fluorescent lamps, 60 to 90 lumens per watt.
LED light sources usually include a plurality of individual LEDs that may be arranged in both a series and parallel relationship. Thus, a plurality of LEDs may be arranged in a series string and a number of series strings may be arranged in parallel to achieve a particular desired light output.
LED light sources are typically rated to be driven via either a current load control technique or a voltage load control technique. An LED light source that is rated for the current load control technique is also characterized by a rated current (for example, 350 mA) to which the peak magnitude of the current through the LED light source is regulated to ensure that the LED light source is illuminated to the appropriate intensity and color. In contrast, an LED light source that is rated for the voltage load control technique is characterized by a rated voltage (for example, 15 V) to which the voltage across the LED light source should be regulated to ensure proper operation of the LED light source. Typically, each string of LEDs in an LED light source rated for the voltage load control technique includes a current balance regulation element to ensure that each of the parallel legs has the same impedance so that the same current is drawn in each parallel string.
It is also known that the light output of an LED light source can be dimmed by using a pulse-width modulation (PWM) technique and a constant current reduction (CCR) technique. PWM dimming can be used for LED light sources that are controlled in either a current or voltage load control mode. In PWM dimming, a pulsed signal with a varying duty cycle may be supplied to the LED light source.
If an LED light source is being controlled using the current load control technique, the peak current supplied to the LED light source is kept constant during an On-time of the duty cycle of the pulsed signal. But as the duty cycle of the pulsed signal varies, the average current supplied to the LED light source also varies to vary the intensity of the light output of the LED light source.
If the LED light source is being controlled using the voltage load control technique, the voltage supplied to the LED light source is kept constant during the On-time of the duty cycle of the pulsed signal in order to achieve the desired target voltage level, and the duty cycle of the load voltage is varied in order to adjust the intensity of the light output.
Constant current reduction dimming is typically only used when an LED light source is being controlled using the current load control technique. In constant current reduction dimming, current is continuously provided to the LED light source, however, the DC magnitude of the current provided to the LED light source is varied to thus adjust the intensity of the light output.
There remains a need in the art for an energy-efficient and simple LED driver circuit to control dimming of an LED light source. with reduced component count.